Wave generating and modulating system



Jan. 13, 1931.

H. 5. BLACK WAVE GENERATING AND MODULATING SYSTEM Filed Sept. 6, 1927 f LUAP CIRCUIT I W BF /NVENTOR fimow 51 BLACK Arr Patented Jan. 13,1931

ran sin-res ears Hew tt HAROLD STEPHEN BLACKQOF 'MoN'TcLAIaNEW JERSEY, AssIeNonmo artram PHONE LABORATORIES, INCORPORATED, or NE YORK, II."Y.,. A oonronATIoN or NEW YORK WAVE GENERATING AND MonULATiNe sYs'rEra Application filed September 6, 1927; Serial No. 217,679.

The present invention relates to systems employing, preferably space discharge devices for producing oscillations, and also to systems for both producing and modulating os- 5 ci-llations; The invention is particularly applicableto systems in which the'space current of the device or devices is permitted to flow for only a relatively small portion of each cycle of the Wave that is being produced, 10 or produced and modulated.

From the functional viewpoint, oscillation producing and modulating circuits may be looked upon as frequencyconverting systems in which the purpose is to produce one 'or more frequencycomponents. The inven tion increases the efliciency of production "of desired components by space discharge devices of given power rating by minimizing the load put on the tubesby unnecessary frequency components, in other WOTClSybYCOH fining the load on the tubes or devices to those components that are most effective in producing the desired final: frequency components, thereby enabling the useful load obtainable with tubes of given power rating to be increased over prior art systems and methods of operation. 1

It has beenfoundin the prior artthat attempts to obtain increased output of de sired "frequency components by increasing the impressed voltages with prior artmethods of operation, that is, 'crowdingl thetubes,

have resulted in irregular operation amount ing in extreme cases to the o'ccurrenceofth'e phenomenon of blocking. The :inventionienables the output of desired frequency components tobe increased while keepingithe operation perfectly stable and 'freefrom irregularities and blocking efiects. I

Ob3ects of'the IIIVGIIUOII'ELIGI e To increase the available output of de-v sired frequency components rin an oscillation producing systemor in anros cill'ation producing and modulating system.

such systems; and v To cont-ro-l the'tube impedance by external circuit adjustments in a novel manner to be more fully described hereinafter.

As a matter of conveniencein :the art a To increasethe stability of operation of distinction has customarily been drawn between modulators and demodulators. However, it' will be simpler in the present case to describe-the invention as applied to a modus lator since so far as theinvention' is con-.

cerned its application to a modulator is'essentially the same as it-sapplicationto a demodulator. In the case "of a'modulator-the applied waves are, in carrier telephony,;for eXa-mpla'speech and the carrier waves while the, desired output f equency components are side hand components and carrier, the latter of which may or may not be actually transmitted although it occurs :in the output with, and is an accompaniment. to the side-hand components. In a demodulator the-applied Waves are a side-band component (o'neor both side-bands) and the carrier, wave'while the output is speech Waves, the carrier that: oc-

ours as an incidental not being used by the receiver. It is noW Well known that the interactions in the circuits are essentially the same in a modulator as those in a demodulator. Wherever in the -follow'ing description andin the' claims,itherefore the term modulator orthelike-is used itwill be undestood that this refers equally asiwell to a demodulator. a I j A a better understanding of the invention willbe had from thQfOllOWlIlg; detailed 'description in connection with the accompanying drawing in'which:

V Fig. lisfa schematic"representation of one a form of oscillator embodying thefinvention, -Fig."2: is a-similar schematic diagram of an oscillator modulator embodying the invention, and

- FigsQ-A and 2B-represent alternativefilter connections to give the-desired terminating iinpefdances in typical casesthat 'willbe described. 1 i

ltis nowwell iniderstood in the art'that a vacuum tube oscillator can be inadeto operate at high efiiciency if the gridzis polarized negatively; with respect tothe cathode'and particularlyto such an extent that thespace current through the tube is allowed to flew for only a; small portioniof each period of the oscillations. One manner of obtaining a negative-biasing potential for the grid is by the use of a parallel condenser-resistance combination, the resistance developing a drop of potential between its terminals due to the fiow of grid current and the condenser serving to maintain the potential drop substantially constant by storing a charge between the instants of grid current flow.

When the space current fiow is confined in this manner toa smallportion ofthe cycle in order'to raise the et'icie'ncygthe actual output power is reduced, other Conditions remaining the same. Attempts to increase the output while still operating with the space current interrupted for the greater part of the cycle as by increasing the driving voltage have resulted in the prior art in irregular or unstable operation of the circuitextending in some instances to actual blocking of the circuit. As is well-known, when blocking occurs,- the oscillationsoccur in successive trains or spurts rather than in a perfectly continuous manner, f

In accordance with the invention the oscillator may be made to give a greatly increased oscillating poweroutput and at the same time remain stable by properly controlling the external circuit impedances as will now be described;

Referring first to Fig. 1, the vacuum tube 1 has in its grid circuit the secondary of the feedback coupling 2, and the grid biasing resistance 3 shunted by condenser 4, and its plate or anode circuit comprises two branches. Onebranch extends through the large impedance 5 and space current source 6 and the other branch extend-s througha number of tuned circuits 7 8 and 9 in parallel, and the resonant circuit 10, the latter being the circuit which exercises the principal frequency control over the oscillations produced. A load circuit 11 may also be coupled to thecontrol circuit 10. Y

Instability in the operation of an oscillator, which in the extreme case takes the form of blocking, consists in a variation in the amplitude of the oscillations at a comparatively low frequency rate. lt -has been proposed heretofore to prevent blocking action by diminishing the average negative charge on the grid, the theory being that the grid periodically accumulates such alarge' negative charge as to choke off or greatly diminish the space current and thus prevent the continuance of production of sustained oscillations. However, in accordance with the invention it is desired to have the grid maintainits high negative charge in order that the space currentmay flow in short quick impulses atthe oscillation frequency and thereby operate at high plate eiiicienc'y. At the same timethe actual oscillatory output may be maintained high if the maximum amplitude of these space current impulses is sufiiciently. high, even if the length of the impulses is extremely short. Sincethe lack mental.

of stability requires that the amplitude of the oscillations vary at a fairly low frequency rate the invention provides for forcibly pre venting the occurrence of variations at a low frequency. rate by making the impedance of the anode-cathode circuitpracticallyv infinite at all frequencies that must necessarily be present in the occurrence of blocking or other unstable actions In order to do this, the path 56 is made of substantially infinite impedance to all frequencies that are low in comparison with the oscillation frequency. The choke coil 5 is given an inductance of a different order of magnitude from that which has been common in the prior art. In a particular instance used by applicant the choke coil 5-had an inductance of the order of 200 henries with a very low ohmic resistance. I

The output branch for the high frequency components in the ideal case would also be made to have infinite impedancev at frequen cieslow compared to the fundamental fre- I quency and particularly at any frequency at mich the circuit would tend to exhibit blocking, and would be made tohave a zero impedance at all of the harmonics of the funda- To realize these conditions in an ideal case would require an infinite number of tuned circuits 7, Sand 9, etc., tuned in succession to the fundamental and each one of the harmonics. This would make the alternatin current branch as a whole" have substantially zero impedance at all of the harmonicffrequencies. The circuit 10 or this circuit together-with the circuit branch 7 should beadjusted tomake the impedance at the fundamental frequency equal to that of the load circuit 11, in order to obtain maximum transfer of oscillatory power into the load circuit. With these impedance conditions the circuit is enabled to develop all of the harmonics that are necessary in order that the space current may flow in extremely short sharp impulses at the fundamental frequency.

In the practical case, of course, the effect of the infinite number of tuned circuits 7, 8'

and9etc., which would berpresent in the ideal circuit can be approached by designing a single network to take the place of the large number of tuned circuits. If 7 represents the fundamental frequency the harmonics higher than 3; are of relatively small importance so that the ideal is closely approached by the use of three tuned circuits resonant at frequencies f, 2f and 3f. 7 A simpler combination of tuned elements than. these three tuned circuits may, however, be used to approach the ideal and the l1m1t1'ngcase would, of course, be a smgle condenser designed as a compromise to give the lowest impedance to the fundamental and its harmonics.

The grid bias resistance 3 andcondenser 4: should be made sufficiently large to insure usages-o the static characteristic, that is, for the greater part of the time the grid is so far negative that there is no space current. :As

the grid potential varies farther and farther in the positive direction there will be an instant of time in each period when the tube 1 conducts space current and a still smaller instant of time when grid current can flow. By the use of sufficiently large values for elements 3 and 4 the grid can be maintained at the requisite negative bias.

In Fig. 2 a pair of similar space discharge devices 20 and 21 are connected in balanced or push-pull relation and are provided with circuit elements2 to 10 inclusive as in the case of Fig. 1 to make the tubes oscillate in parallel.

The grids of the tubes 20 and 21 are differentially connected to the incoming line LL and the plates are differentially connected to the outgoing line HL. Assuming that the incoming line transmits speech or other low frequencies, a low-pass filter LF is inserted in the line LL ahead of the input transformer 22. A band filter BF preferably designed to transmit a single sideband resulting from the modulating action of the circuit is inserted in the line HL following the output transformer 23.

As was explained heretofore the circuit 7 may be used either as a modulator or a demodulator and it could just as well be assumed that the waves impressed through the input transformer 22 from theline LL r'epre: sent an incoming sideband'tobe demodulated and that the waves impressed upon the line HL are demodulated signal waves such as speech. In the latter case the filters LF and BFwould, of course, be suitably designed to pass the necessary frequency components. 1

It was pointed out in connection with Fig. 1 that the impedance of the anode-cathode circuit at the fundamental frequency should be made equal to the impedance of the load while at frequencies low with respect to the fundamental the impedance should be practically infinite and at harmonic frequencies the impedance should be zero. These same impedance relations hold for the modulator circuit of Fig. 2 except that in the case of the modulator it is desired that the oscillatory energy developed in the circuit be dissipated in the tubes 20 and 21 rather than ina load circuit such as 11 of Fig. 1 since this circuit is a modulator and the purpose is to produce as large a distortion as possible in the fundamental or carrier oscillators. The anode-tocathode impedance at the carrier frequency will therefore depend upon the type of modulation that is used as will be explained hereinafter.

It was pointed out in U. S. Patent 1,448,702 to Carson dated March 18, 1923 t iat where modulation is efiected by. virtue ofithe non-linear relation between the space.

current andv the grid potential of a tube the efiiciency of the circuit for modulation is maximum when the anode-cathode impedance is made zero at the; carrier frequency e and atthe frequency of the impressed modulating waves. It has been found that if this impe ancefis made zero, also at all frequencies higher than the carrier, and also including the unwanted side-band in case only one side-band is to be transmitted, the efficiency of the circuit a modulator is increased to pronounced degree. This type of modulator is conveniently termed in the art a plate current modulator since the distortion which produces moduiation is produced by the control of the grid on the space. current of the tube. A second type of modulation. that is well, known in the art is termed grid current modulation which means currentis'perl'nitted to flow in the grid circuits and modulation is effected by distorting this grid current. T grid potential varies corresponding to modulated grid current and, preferabiywithout further distortion, correspondingvariations are setup in the plate current. It has been shown that the efficiency o'fa circuit in producing grid current modulation is a maximum when the impedance of tie out oin circuitis made equal to the totai anode-to-cathode impedstated, the impedance looking to the left of thegtubes 20 and 21 should be made equal to the. impedance of the-incoming circuit for the impressed waves including both voice and carrier, and preferably Zero for'all other components. The impedance looking to the right of tubes 20 and 21 should be made circuit for the impressed waves including both voice and carrier and should be zero for ail other frequencies. Preferably, this impedance looking to the left should also be substantially infinite for the wanted sideband. The impedance looking to the right of the tubes 20 and 21 shouldmatch the outgoingcircuit for the 'sideband.

The above stated impedance relations may be summarized as shown in the following.

Plate Grid current current modulator modulator Frequency components Input circuit impedaneesz.

Speech (modulating waves) Matched" Matched. Carrier and its harmonics; Matched" Matched.

' output Wanted side 'band. modulation Infinity. Zero.

components Zero Zero.

All other frequencies Output circuit impedances:

output Wanted side band... modulation Matched- Matched.

components All other frequencies Infinity. Zero.

It will be understood that the above relations are the ideal or optimum relations and that in practice they can only be approached but not fully realized, except in a practical as distinguished from a theoretically ideal sense. The sideband components may coinprise either a. single sideband on both sidebands, based on either the carrier or fundamental frequency, or harmonic frequencies or both. y

The impedance of the circuit at the carrier frequency is determined in the manner already described in connection with Fig. l and by the same respective tuning elements as in the case of Fig. 1. Due to the fact, however, that the tubes are differentially connected for the speech and sideband components, when the transformers 22 and '23 are balanced the impedance looking to the left and that looking to the right at all frequencies except the carrier frequency and its harmonics may be considered to be the impedance offered by the filters LF and BF respectively'since each transformer 22 and 23 has substantially no total impedance on account of its differential connection. The impedance at the carrier frequency and its har monics is independent of the filters since these components go through the two halves of each transformer in opposite directions.

The terminal impedances of the filters may be controlled in manners well understood in the art of filter design. For the'purpose of illustration simple types of filters suitable for both types of modulator circuits are shown. In Fig. 2 the filters LF and BF are terminated suitably for a plate current modulator, their shunt elements being adjacent the modulator input and output respectively so that by proper design these filters may readily be made to fulfill the impedance requirements above stated for this type of modulator'circuit. I

If the circuit of Fig. 2 is to operate as a grid current modulator a suitabletype of filter termination is illustrated in Figs. 2-A and 2-13, it being understood that the circuit to the left of the dotted line AA in Fig. 2 is to be replaced by thecircuit of Fig. 2A and the portion of the circuit to the right of line BB is .to be replaced bythe circuit of Fig 2B. l Vith the proper design of the filters LF and BF of Figs 2A and 2B, impedances to the speech, the sideband components and all higher frequencies corresponding to those above outlined may readily be obtained. v r

. Coming. now to the impedance requirements in the oscillator part of the circuit, if the circuit is to be used as a plate current modulator the total cathode-to-anode impedance should be substantially zero at' the carrier frequency and should be the same as in the case of Fig. l for all the other frequency components. In the case of a grid current type of modulator, however, the cathode-anode impedance at the carrier frequency should be as high as practicable and in the ideal case would be infinite while the impedance at all of the other frequencies would be as in the case of Fig. 1.

It has been found in practice that when these impedance relations are observed a circuit of the type shown in Fig. 2 used either as a plate current or a grid current modulator (or demodulator, of course) is remarkably stable in its operation and can be made to produce a. very much greater output of desired modulated energy for the same type of tube and sameexpended energy than has been found to be possible in circuitsv of the prior art. One reason for this .is that the circuit may be overloaded to'a much higher degree than circuits of the prior art and when so overloaded shows no tendency to block or become unstable. As in the case of-Fig. 1 the tuned circuit branches 7, 8 and 9 of Fig.

"2 may in practice be replaced by a simpler network which in the'limiting case will-take the form of a condenser designed as a compromise to have as nearly as possible the ideal impedance at the fundamental and all of the harmonics.

iVhen either the single tube circuit of Fig. 1 or the balanced circuit of Fig. 2 is adjusted to have its space current zero for a large part of each cycle, and is used as an oscillator or modulator, for example, it has been observed that the impedance of the tube or tubes can be controlled in a marked degree by varying the impedance of the alternating current branch to the higher frequencies. For example, in Figs. 1 and 2 (on the assumption that in both cases the space current is zero V understood that this in the internal anode-to-cathode impedance of the tube or tubes rises to a high value. The

maximum amplitude of the oscillations of fundamcn till frequency also falls off but this can be cc mp en ated for by increasing the ratio of the feedback coupling 2. ihe tube thus tallies up a new condition of oscillation with a different average tub-e impedance. This phenomcnom is useful in certain situations such, for example, as measuring circuits Where a given type of tube may be controlled to have different internal impedances With the same amplitude of oscillations.

Where two modulating devices are used as shown in Fig. 2, there are two incoming circuits, one through'the transformer 22 for the speech or other modulating. Waves and the other through the transformer 2 for the carrier Waves. Where reference is made in the claims to an incoming circuit it Will be relate to a coms, in fact'to any men or to separate c5:

circuit by which Waves are impressed on the modulating system, including other types of connection than those shown, several of which are familiar in the modulator art.

The impedance relations disclosed are not limited to the modulator circuits disclosed but are general and the scope of the invention is to be limited only by the claims. Other modulating elements or devices than space discharge devices may be used inaccordance With the broader aspects of the invention such, for example, as magnetic systems or asymmetrically conducting devices or rectifiers.

lVhat is claimed is a 1. The method of operating an oscillator of the type using a space discharge device having a grid or control element Whose potential is varied at the oscillation frequency about a value so far negative With respect to the cathode that the anode current is interrupted for a large portion of each period of the oscillations, which method comprises making the total external impedance as measured between the anode and cathode very low and approaching ZBIO for multiple frequencies of the oscillations produced, and very high and approaching infinity for all frequencies substantially lower than the os-' cillati-on frequency. q

2. An oscillation producing circuit comprising a space discharge device having a cathode, a grid and an anode, grid and anode circuits, a tuned circuit relating the grid and anode circuits, means in the grid circuit for biasing the grid so far negative with respect to the cathode that the space current is zero for a large portion of each period of the oscillations, a path of practically infinite impedance to low frequencies for supplying direct current to the anode, and impedance meansinterposed between the tunedcircuit and one of the output terminals of thedischarge device, said last means being substantially equivalent in impedance to as many tuned circuits-in parallel as there areharmonics of the fundamental. oscillations present, eachtuned to a respectiveharmonic, Wherebythe total anode to cathode external impedance is of the orderof infinity for all frequencies low in comparison Withthe fundamcnt nionic"frequencies. V

An osclllat ng modulatingsystem comprising a pair ofthi'ee-element discharge devices coiinected in balanced relation, a com mon anode-cathodecircuit branch for said devices, differential; circuit connections for applying input WllVQSlZO the system and, deriving modulationcomponents from the systemia {tuned feed-back circuit branch, cou- .pli;ngthe anode circuits in parallel to thegrid circuits in parallel for enabling the system to produce oscillations of carrierfrequency, and impedance means ,ini said circuit branches for making the total external cathode-anode impedance of. the system practically infinite. at. frequencie'sv'lovv incomparisonwith the carrier frequency, and mad tically zero at frequenciesharmonic to the carrier. 13 i ,v

.4. A system inaccordance Withclaim 3, in which :the cathode-anode impedance at the carrier frequency and at the frequencies of the input modulating componentsis substantially Zero, ,lvvhereby said circuit acts eficiently as self-oscillating platecurrent modulaton: f 3 7 1 5. A; ystem according to claim 3, in Which thefcathodeganode impedance at the carrier modulated v A Wave modifying system having an 7 input and an output circuit, incoming and outgoing circuits, and means to apply to said land practically zeroat the har "frequencyandatthe frequenciesaof the input system Waves tobe modulated and modulating aves, the impedance of the output circuit of said system being matched tothat of the outgoing circuit for the modulated Wave components to be transmitted, and approachiingfzero at the frequencies of the applied waves, and the impedance of the input circuit being matched to that of the incoming circuits forthe applied Waves, and being of the order of zero or negligibly small at all otherjfrequenciesi' 3 p r 1 ;"7 l wave modifying system having an, nput and an,outputxcircuit, -incoming and outgoing circuits, and means toiapply to said syste'm waves to be "modulated andfmodulat- 111g Waves, the impedance of the output circuit of said system being matched to that of A the outgoing circuit for the modulated wave impedance being substantially zero at the components to be transmitted, and approaching infinite value at the frequencies of the applied Waves, and the impedance at the input' circuit being matched to that of the incoming circuits for the impressedwaves and approaching infinity at all other frequencies.

8. A system according to claim 6 in which an output-to-input coupling is provided for making the system self-oscillatory at a fundamental frequency, the total anode-cathode fundamental frequency, and its principal harmonics, and practically infinite at all charge device comprising applyingto the frequencies low with respect to the funda-. mental, other than the sideband. 9. A system-according to claim 7 in which an output-to-input coupling is provided for making the system self-oscillatory at a fundamental frequency, the total anode-cathode impedance beingpractically infinite at the fundamental frequency and at all othe frequencies except the sideband. g

10. In an oscillator employing a three-elemerit discharge device in which the space current is zero for a large portion of each period of the oscillations, the method of controlling the internal tube impedance independent of conditions in the grid circuit comprising varying the total external anodecathode impedance withinv a region appreaching practically infinite value at all frequencies except the oscillation frequency and correspondingly varying the amplitude of the wave fed back tothe grid. r

- 11. The method-of controlling the internal anode-cathode impedance of a space disgrid a negative bias potential and waves such that the space current flows in only fraction- I alparts of the fundamental oscillation cycles, and controlling the total external cathode; anode impedance at substantially all frequencies present except the fundamental,

while at the same time controlling theamplitude of the'said applied wave to maintain the amplitude of the fundamental space current impulses substantially unchanged.

12. The method of controlling the production of combination wave components of impressed waves'in a Wave repeater having an outgoing circuit therefor, comprising making the impedance looking intotheoutgoing circuit practically zero at the frequencies of all componentswhich are produced but are not to be transmitted, practically infinite at the frequencies. of all components which it is desired not to produce and matching the impedance looking intothe outgoing circuit to that of therepeater for the frequency com.- ponents that are to betran'smitted.

In Witness whereof,'I hereunto subscribe my'7name this'lst-daypf September, A. D.,

- 'HABO LD STEPHEN BLACK. 

